Magnetic resonance imaging is capable of producing images of inner structure of matter. MRI is frequently used in hospitals and other medical or research facilities for diagnosing diseases and for research purposes. Remarkable exemplary areas of use are cancer diagnosis and brain research. ULF MRI has recently shown potential in these areas due to its excellent contrast between different kinds of tissues, like healthy and cancerous tissue.
While the state of the art of MRI has developed into multiple-tesla scanners, another approach has emerged, where the signal is measured in a magnetic field on the order of 100 μT. In ULF MRI, NMR occurs at corresponding low frequencies in the kilohertz range.
Prepolarized MRI is based on magnetizing a sample with a prepolarizing magnetic field and, for instance, measuring relaxation of the magnetization precession in a magnetic field that is typically lower than the prepolarizing field. Prepolarization in the stronger field is needed in order to achieve an acceptable signal to noise ratio (SNR). Each position in the sample to be imaged can be encoded using gradient magnetic fields such that, e.g, a 3D image of the sample can be mathematically reconstructed from the measured signals.
In addition to MRI, the prepolarization technique can be used, for example, in magnetic nanoparticle imaging (MNI) or in magnetorelaxometry (MRX). In MNI, the target is imaged with the help of small particles with suitable magnetic properties, which are generally administered into the object under study. In MRX, the magnetic relaxation properties of the target can be measured with or without the aim of forming an image of the sample. There are also other imaging and non-imaging measurement techniques utilizing prepolarization.
ULF MRI can be combined with magnetoencephalography (MEG), which utilizes ultra-sensitive SQUID sensors for detecting physiological electric currents in the human brain.
In order for the image quality to remain high despite the low main field B0, the sample to be imaged needs to be prepolarized in a stronger magnetic field Bp, typically of the order of 10-200 mT, before the weaker, typically homogeneous, field B0 and the gradient fields are applied for signal encoding. When the signal is measured with an untuned SQUID sensor, its amplitude is independent of B0 and proportional to Bp. In ULF MRI, signal-to-noise ratio (SNR) is typically a limiting factor; thus, the imaging time for a given spatial resolution (voxel volume) depends on the SNR as 1/SNR2, which is proportional to (BN/Bp)2, where BN is the noise standard deviation. Therefore, the polarizing field should be as high and the noise level as low as possible to obtain high-quality images in a short imaging time.
Changing magnetic fields induce eddy currents in conducting structures nearby. The eddy currents cause magnetic interference in the sample and measurement equipment and may therefore disturb the measurement. This effect is of particular importance in combined MEG-MRI, as the detectors are ultra-sensitive and the measurements are done in a magnetically shielded room. Even if the detectors would not be disturbed, the eddy currents may be detrimental as they may destroy the spin dynamics in the sample to be imaged and set a practical limit for the strength and switch-off time of the prepolarizing field. In early studies, this has had the effect that the prepolarizing coils have been designed to be small and therefore not suitable for brain research, for example.
BN can be reduced by providing shielding against external noise sources. For ULF MRI at the kilohertz range, a light magnetically shielded room (MSR) having an aluminum layer around the measurement system together with gradiometric sensors are enough to render the external noise insignificant. If ULF MRI is combined with MEG, additional shielding at lower frequencies is usually needed. Such a shielding can be achieved with an MSR consisting of a few layers of mu-metal with high permeability, together with thicker layers of aluminum.
However, combining the shielding and high prepolarizing fields is problematic. Because the prepolarizing field has to be switched off rapidly, a large time derivative ∂Bp/∂t appears. With dipolar polarizing coil designs typically used, a strong stray field occurs with Bp. Thus, when a typical polarizing field is switched off, strong eddy currents are induced in the conductive layers of the magnetically shielded room. The induced eddy currents decay in a multi-exponential manner depending on the resistances and inductances of the conductive paths. The eddy currents cause secondary magnetic fields inside the MSR. If these fields are strong, they will affect the spin dynamics of the sample, reducing image quality, or, in the worst case, making image reconstruction practically impossible. A large drifting magnetic field may also exceed the dynamic range of the sensors.
In addition, such fields typically contain low-frequency components; thus, they may interfere with MEG recordings, frustrating simultaneous MEG-MRI.
The influence of eddy currents has been given attention to in high-field MRI, where the source is the rapid switching of the gradient fields. Eddy currents affect the field homogeneity and stability, producing image artifacts.
One of the most widely used techniques for minimizing eddy current-induced artifacts is to design gradient coils with weak stray fields. Low stray field gradient coils can be designed using numerical or analytical design methods. The patent publications U.S. Pat. No. 5,561,371, GB 2265986, EP 0749017 and U.S. Pat. No. 4,733,189 describe exemplary arrangements for high-field MRI, which have been designed to minimize the stray field of gradient coils.
These methods are not directly applicable to the prepolarizing coil in ULF MRI, prepolarized MRI, ULF NMR, prepolarized NMR, MNI, MRX, or, in particular, ULF MRI combined with MEG.
M. Burghoff et al., “SQUID systems adapted to record nuclear magnetism in low magnetic fields”, IEEE Trans. Appl. Supercond., 17:846-849, 2007, disclose a system where a tiny solenoid is used to polarize the sample; an equivalent coil, with an opposite field direction, is placed beside the polarizing coil to reduce the stray field and to prevent the magnetization of the MSR walls. However, such a design becomes impractical when the sample volume and the strength of the polarizing field increase.
U.S. Pat. No. 4,978,920 discusses, in general level, screening of a magnetic field using the concept of “hypothetical superconductive shield” neutralizing the main field. The discussed geometry is, however, not suitable for prepolarized ULF MRI or MRX, for example, in which the prepolarizing coil is typically a solenoid or other coil whose dipole moment is very large. Also the current required in the prepolarization coil is very large, whereby they produce a very strong magnetic field to the surrounding structures, including the MSR walls.
It is also known to rotate the sample during the measurement in order to raise the measurement frequency and further in order to be able to distinguish between disturbances from the surroundings of the measurement system and real signals arising from the sample. This approach is, however, quite impractical to implement in practice. Even more inconvenient, but in theory possible, would be to carry out the polarization and measurement of the sample at different locations.
Thus, there is a need for achieving more convenient and efficient methods and arrangements for minimizing the effect of prepolarizing-field-induced eddy currents in magnetic measurements.